1. Field of the Invention
The invention relates to a transistorized tuned-circuit oscillator, especially in a proximity switch, having a current source, which is arranged in the circuit of the tuned-circuit transistor, is controlled by a temperature sensor, which drives the current source high with rising temperature and vice versa in order to stabilize the tuned-circuit voltage.
2. Description of Related Art
It is known that the oscillation behaviour of tuned-circuit oscillators depends on the ambient temperature. Inductive proximity switches which have a tuned-circuit oscillator require a high sensitivity, because otherwise the switch does not react sufficiently precisely to small changes in position of the target to be detected. Owing to the temperature dependence of the tuned-circuit coil, there is a change in the resonant impedance of the tuned circuit, with the result that the switching points of the proximity switch are not temperature-stable. It has been attempted to compensate for this temperature dependence by means of a temperature-dependent resistor (NTC resistor) arranged in the feedback branch of the tuned-circuit transistor. Such a temperature compensation has a plurality of disadvantages:
a) Compensation by means of the temperature-dependent resistor is not optimum, because the temperature-dependent impedance characteristic of the tuned-circuit coil is linear, whereas the resistance characteristic of the temperature-dependent resistor obeys an e function.
b) At +/-20%, the initial tolerances of the temperature-dependent resistor are quite large, and its long-term stability is relatively poor.
c) The temperature-dependent resistor is situated in the RF-carrying feedback branch and therefore has to be arranged in the immediate vicinity of the tuned-circuit oscillator. Thermal coupling to the coil, which is the precondition for good temperature compensation, would therefore lead to resistor connecting leads too long for RF.
d) The generally customary trimming resistor for setting the switching interval, which is in series with the temperature-dependent resistor, influences the compensation effect of the temperature-dependent resistor.
e) Since the trimming resistor carries RF, it must likewise be arranged in the vicinity of the tuned-circuit oscillator. Consequently, it cannot be arranged with long connecting leads at the rear end of the tuned-circuit oscillator, which s constructed as a hybrid circuit, which would be favourable for simple laser trimming.